1. Field of the Invention
The present invention relates to an apparatus for controlling the amount of slip of a lock-up clutch provided in a power transmitting system of a motor vehicle.
2. Discussion of the Related Art
In a motor vehicle having a fluid-filled power transmitting device equipped with a lock-up clutch such as a torque converter or fluid coupling incorporating such a lock-up clutch, it is proposed to control the lock-up clutch in a slip control mode (partially slipping or engaging mode) such that an actual amount of slip (i.e., slip speed) of the lock-up clutch, namely, a difference between the speeds of a pump impeller and a turbine impeller of the power transmitting device eventually coincides with a predetermined target slip speed, for the purpose of improving the fuel economy of the vehicle by minimizing the power loss due to slipping of the lock-up clutch while preventing locking tendency of the vehicle engine and deterioration of drivability of the vehicle, and by maximizing a time duration for which the fuel supply to the engine is cut during deceleration or coasting of the vehicle. The slip control mode is established when the running condition of the vehicle is in a predetermined slip control area which is intermediate between a fully releasing area in which the lock-up clutch should be held in a fully released state, and a fully engaging area in which the lock-up clutch should be held in a fully engaged state. These fully releasing, fully engaging and slip control areas are defined by suitable parameters (e.g., throttle valve opening and vehicle running speed) indicative of the vehicle running condition.
Generally, the slip control of the lock-up clutch is implemented by a feedback control device of so-called speed-control type or differential-integral type, which uses a feedback control equation formulated such that a slip control output of the feedback device includes a control error or an integral action component of the slip control output. This feedback control device is adapted to control a hydraulic system which uses a hydraulic power source that permits full engagement of the lock-up clutch. The hydraulic system has a releasing oil chamber and an engaging oil chamber which are formed on the opposite sides of a piston of the lock-up clutch so that the piston is movable between fully released and fully engaged positions of the clutch, depending upon a pressure difference of the two oil chambers. The feedback control device is arranged to control the pressure difference of the two oil chambers for changing a force acting on the clutch piston to thereby change a friction force of the lock-up clutch and the amount of engagement or slip or the slip speed of the clutch. In this feedback control arrangement, even a small amount of change in the hydraulic pressure difference, that is, even a small amount of change of the slip control output of the feedback control device will cause a considerable amount of change of the slip amount or slip speed of the lock-up clutch, since the friction force of the clutch is inherently comparatively unstable with respect to a change in the force acting on the clutch piston. In other words, the amount of slip of the lock-up clutch is sensitive to a change in the value of the slip control output of the feedback control device. Thus, the feedback control of the slip amount of the clutch is comparatively unstable. In view of this drawback, the slip control equation used by the feedback control device is formulated to determine the slip control output of the feedback control device on the basis of a feed-forward control value, a learning control value and a feedback control value. The feed-forward control value corresponds to the currently detected output of the vehicle engine. The learning control value is updated depending upon the vehicle running condition, so as to compensate for a deviation or offset of the feedback due to fluctuation of the operating characteristics and chronological change of the lock-up clutch, for thereby improving the accuracy of the feed forward control. The feedback control value is determined to zero the control error, that is, to control the above-indicated hydraulic pressure difference to establish the target slip speed of the lock-up clutch. An example of such a slip control equation used for feedback control of the lock-up clutch is disclosed in JP-A-4-203561. According to the feedback control equation disclosed therein, the ratio of the feed-forward control value and learning control value to the total output value is higher than that of the feedback control value, and the influence of the feedback control value on the slip control is constantly made smaller. Further, the feed-forward control and the learning control are effected in advance of the feedback control. The feed-forward control is supplemented or compensated by the learning control. Thus, the slip control equation is effective to prevent control instability of the feedback system due to delayed response and compensate the slip control output for possible fluctuation of the operating characteristics of the lock-up clutch.
In the conventional slip control apparatus wherein the learning control value included in the slip control equation plays an important role, the feedback control stability and accuracy may be deteriorated in connection with the learning control, causing an adverse influence on the vehicle drivability and fuel economy. In this sense, the conventional slip control apparatus has some room for improvement. In a discrete time control system, for example, learned values corresponding to respective areas or ranges of a selected vehicle running condition parameter (e.g., throttle valve opening or turbine impeller speed) are updated during the slip control cycles. The learned values which are updated from time to time are stored in a memory in relation to the respective ranges of the vehicle running condition parameter. The appropriate learned value corresponding to the current vehicle running condition is read out from the memory to determine the learning control value at a predetermined cycle time. It is desired that the learning control value be updated in a shorter time to suit the specific operating characteristics of the lock-up clutch, to attain consistent improvement in the fuel economy of the vehicle, irrespective of a deviation of the clutch operating characteristics from the nominal characteristics. To this end, it is considered to quickly increase the ratio of the learning control value to the feedback control value in the feedback control equation. However, this leads to an increase in the amount and rate of change of the learning control value, which may cause control disturbances resulting in deterioration of the feedback control stability.
As indicated above, the different learned values used for determining the learning control value are stored in a memory in relation to respective ranges of the vehicle running condition parameter. If the number of these ranges is small, the number of the learned values available is accordingly small. In this case, the selected one of the learned values does not necessarily suit the specific vehicle running condition. If the number of the learned values available is relatively large, the learned values corresponding to some of the ranges of the running condition parameter may not have been updated during the slip control cycles before these learned values are used. Thus, it is inevitable that some of the learned values used do not accurately reflect the actual characteristics of the lock-up clutch. In any event, the learned values stored in the memory tend to deviate from the actual vehicle running condition and the characteristics of the lock-up clutch, and the slip control of the clutch cannot be achieved with high accuracy. If, for example, the learning control value used during acceleration of the vehicle deviates from the optimum value in the engaging direction of the clutch, the slip speed of the clutch is lowered, causing a knocking tendency of the vehicle engine. If the learning control value used during deceleration of the vehicle deviates from the optimum value in the releasing direction of the clutch, the friction or engaging force of the clutch is insufficient, resulting in lowering the engine speed and reducing the time duration of the fuel-cut period of the engine, which undesirably reduce the fuel economy.